1. Field of the Invention
The present invention relates to a method of producing a semiconductor device by cutting out separate pieces of semiconductor chips from a semiconductor wafer, and a grinding machine for use in the method of producing the semiconductor device.
2. Description of Related Art
Production processes of a thin semiconductor device such as a semiconductor device for IC chips include a grinding process for grinding an inactive surface (back surface) of a semiconductor wafer with a grinder. This grinding process is carried out prior to a dicing process for dicing a semiconductor wafer to divide it into semiconductor chips. This is because if the grinding process is carried out after the dicing process, the back surface of each semiconductor chip has to be separately ground, which causes the grinding process to take labor.
However, cutting such a thin semiconductor wafer with a dicing saw causes cracking of the semiconductor wafer and chipping of semiconductor chips. For this reason, a semiconductor wafer cannot be made thinner than a limit thickness, which is about 50 xcexcm, by grinding prior to the dicing process.
Accordingly, carrying out the dicing process prior to the process of grinding the back surface of the semiconductor wafer has been recently proposed. That is, after an active surface of a semiconductor wafer is cut with a dicing saw so as to form grooves, a tape for protecting the surface is stuck to the surface of the semiconductor wafer. In this condition, the back surface of the semiconductor wafer is ground by a grinder. This back surface grinding is continued until the surface of the semiconductor wafer subject to grinding (back surface) by the grinder reaches the grooves, when thinned separate pieces of semiconductor chips can be obtained.
The arrival of the surface subject to grinding at the grooves may be detectable based on, for example, a change in a torque current flowing in a motor for driving the grinder. That is, when the surface subject to grinding reaches the grooves, the contact area between the back surface of the semiconductor wafer and the grinder decreases, causing the torque applied from the back surface of the semiconductor to the grinder to decrease. As a result, the quantity of the torque current flowing in the motor for driving the grinder lessens, according to which the arrival of the surface subject to grinding at the grooves maybe detected. Additionally, when grinding by using a chemical is performed as in the case of the CMP (Chemical-Mechanical Polishing) process, the arrival of the surface subject to grinding at the grooves may be detected based on a change in the pH level of the chemical.
However, since the change in the torque current or that in the pH level due to the arrival of the surface subject to grinding at the grooves is so little that it is impossible for both of the above-mentioned methods to precisely detect that the surface subject to grinding has reached the grooves. Accordingly, terminating the back surface grinding in response to the detection of the arrival of the surface subject to grinding at the grooves by such methods tends to lead to insufficient or excessive grinding, causing the produced semiconductor chips to scatter in the thickness.
In addition, as another method for detecting the arrival of the surface subject to grinding at the grooves, the method as shown in FIG. 10 is also possible. In this method, a gaugel G is pressed against the surface of a semiconductor wafer W subject to grinding to measure (monitor) the thickness between a wafer stage 91 on which the semiconductor wafer W is mounted and the surface of the semiconductor wafer W subject to grinding so as to determine that the surface subject to grinding has reached the grooves when the thickness of the semiconductor wafer W has become a predetermined value. However, due to unevenness in thickness of a surface protection tape 92 stuck to the surface of the semiconductor wafer W, the results of the measurement by the gauge G include a significant margin of error. Accordingly, it is impossible to obtain a desired thickness e.g. 50 xcexcm of the semiconductor chips with high accuracy.
It is an object of this invention to provide a method of producing a semiconductor device capable of preventing occurrence of cracking or chipping in semiconductor chips as well as suppressing variation in the thickness of the semiconductor chips, and a grinding machine for use in the production method of this semiconductor device.
The method of producing a semiconductor device according to this invention comprises: a groove forming process for forming a groove with a pattern according to an outer contour of a desired semiconductor chip in a front surface of a semiconductor wafer; a wafer holding process following the groove forming process for holding, with a wafer holding mechanism, the front surface of the semiconductor wafer, for instance, by sticking a wafer holding mechanism to the front surface of the semiconductor wafer; a back surface grinding process for grinding a back surface of the semiconductor wafer being held by the wafer holding mechanism by a grinder; and a finish timing determining process for detecting opening of a bottom face of the groove during the back surface grinding process and determining the timing for finishing the back surface grinding process based on the detection.
The opening of the bottom face of the groove (penetration) may be detected by an electromagnetic wave sensor for detecting electromagnetic waves including light or microwaves that pass through the groove. In addition, the method may include a back surface grinding process for grinding the back surface of a semiconductor wafer being held by the wafer holding mechanism, an air sucking process for sucking air inside the groove during the back surface grinding process, and an air pressure monitoring process for detecting and monitoring air pressure inside the groove during the air sucking process, thereby detecting the penetration of the groove based on a change in the air pressure detected during the air pressure monitoring process.
The grinding machine for carrying out the production method of this invention may be one which comprises a wafer holding mechanism for holding the front surface of the semiconductor wafer, for example, by adhering to the front surface of the semiconductor wafer, a grinder for grinding the back surface of the semiconductor wafer being held by the wafer holding mechanism, a penetration detecting mechanism for detecting opening of a bottom face of the groove formed in the semiconductor wafer being held by the wafer holding mechanism, and a control section for determining the timing for finishing the process of grinding the back surface of the semiconductor wafer by the grinder, which determination being made based on-an output from the penetration detecting mechanism.
The penetration detection mechanism may include an electromagnetic wave sensor for detecting an electromagnetic wave such as light or microwave that passes through the groove.
According to this invention, by grinding the back surface of a semiconductor wafer after forming grooves in the semiconductor wafer by dicing, the semiconductor wafer is divided into individual semiconductor chips. Accordingly, cracking or chipping of the semiconductor chips does not occur during the dicing, and extremely thin semiconductor chips being 50 xcexcm or less in thickness can be produced in good order.
Meanwhile, the timing for finishing the back grinding of the semiconductor wafer is determined, for example, based on an output of an electromagnetic wave sensor such as a light sensor.
A light sensor may be disposed in the wafer holding mechanism or in the grinder so as to detect light, which is directed from a light source disposed in the grinder or the wafer holding mechanism and passes through the groove. In this case, when the surface of the semiconductor wafer subject to the grinding by the grinder reaches the groove and the groove comes into a state of penetrating the semiconductor wafer from the surface to the back surface thereof, the light from the light source passes through the groove and enters into the light sensor. Accordingly, the timing at which the light sensor detects the light from the light source corresponds to the timing at which the surface of the semiconductor wafer subject to grinding reaches the groove. Based on the detection of the light that has passed through the groove by the light sensor, the timing for finishing the back surface grinding of the semiconductor wafer is determined so that the back surface grinding of the semiconductor wafer can be always finished with almost constant timing. The amount of the back surface of the semiconductor wafer to be ground is therefore prevented from varying, and the thickness of semiconductor chips obtained from the back surface grinding is kept almost constant.
In this case, it is also possible for the control section to determine the timing such that the back surface grinding of the semiconductor wafer by the grinder is finished at a time when a prescribed time elapses after the light sensor detects the light passing through the groove.
Also, the light sensor may be held by the wafer holding mechanism and of a light emitting/receiving type that directs light toward the groove in the semiconductor wafer held by the wafer holding mechanism while detects light which returns after being reflected by the semiconductor wafer or the grinder.
Due to differences in material and surface configuration between the semiconductor wafer and the grinder, the reflectance of the semiconductor wafer and that of the grinder are different. Accordingly, when the back surface grinding of the semiconductor wafer by the grinder is advanced until the surface of the semiconductor wafer subject to the grinding reaches the groove so that the groove comes into a state of penetrating the semiconductor wafer from its front surface to the back surface, the quantity of the light coming back to the light sensor changes significantly. Here, the timing of the change in the quantity of the light received by the light sensor corresponds to the timing of the arrival of the surface of the semiconductor wafer subject to the grinding at the groove. Accordingly, by determining the timing for finishing the back surface grinding of the semiconductor wafer based on a change in the light quantity received by the light sensor, the back surface grinding of the semiconductor wafer can always be finished with almost constant timing. The amount of the back surface of the semiconductor wafer to be ground is therefore prevented from varying, and the thickness of semiconductor chips obtained from the back surface grinding is kept almost constant.
When using a microwave sensor as the electromagnetic wave sensor, the arrangement may be such that the microwave sensor detects a microwave, which passes through the groove formed in the semiconductor wafer after being directed toward between the grinder and the back surface of the semiconductor wafer held by the wafer holding mechanism from a microwave oscillator which is provided in association with the grinder. In this case, when the surface of the semiconductor wafer subject to the grinding by the grinder reaches the groove and the groove comes into a state of penetrating the semiconductor wafer from its front surface to the back surface, the microwave directed toward between the grinder and the back surface of the semiconductor wafer passes through the groove and reaches the microwave sensor. Here, the timing of the detection of the microwave by the microwave sensor corresponds to the timing of the arrival of the surface of the semiconductor wafer subject to grinding at the groove. Therefore, by determining the timing for finishing the back surface grinding of the semiconductor wafer based on the detection of the microwave having passed through the groove by the microwave sensor, the back surface grinding of the semiconductor wafer can always be finished with almost constant timing. The amount of the back surface of the semiconductor wafer to be ground is therefore prevented from varying, and the thickness of semiconductor chips obtained from the back surface grinding is kept almost constant.
It is also possible to locate the microwave sensor in either one of the wafer holding mechanism and the grinder and to locate the microwave oscillator in the other one of the wafer holding mechanism and the grinder so that the microwave sensor detects the microwave that is directed from the microwave oscillator and passes through the groove formed in the semiconductor wafer held by the wafer holding mechanism.
In addition, it is possible for the control section to determine the timing such that the back surface grinding of the semiconductor wafer by the grinder is finished when a prescribed time elapses after the microwave sensor detects the microwave passing through the groove.
The penetration detecting mechanism may include an air sucking mechanism for sucking air inside the groove formed in the semiconductor wafer held by the wafer holding mechanism and an air pressure detecting mechanism for detecting the air pressure inside the groove. In this case, the control section should preferably determine the timing for finishing the back surface grinding of the semiconductor wafer by the grinder based on an output from the air pressure detecting mechanism.
For instance, the air sucking mechanism may comprise an air inlet formed in the wafer holding surface (the surface contacting with the front surface of the semiconductor wafer) of the wafer holding mechanism, a suction pipe communicating with the air inlet, and a vacuum source connected to the suction pipe. In this case, in a state where the groove has not appeared in the back surface of the semiconductor wafer, the groove on the side of the back surface of the wafer are closed and the air pressure inside the groove is therefore relatively low. On the other hand, in a state where the back surface grinding of the semiconductor wafer has been advanced so that the groove appears in the back surface of the semiconductor wafer, the groove penetrates the semiconductor wafer from its surface to the back surface. Thus, air enters from the side of the back surface of the semiconductor wafer into the groove, by which the air pressure inside the groove suddenly increases.
Accordingly, by monitoring the air pressure inside the groove, especially changes thereof, the arrival of 1S the surface of the semiconductor wafer subject to grinding at the groove can be precisely detected. Thus, by determining the timing for finishing the back surface grinding of the semiconductor wafer based on the detections the back surface grinding of the semiconductor wafer can always be finished when the semiconductor wafer has been ground in an approximately constant amount, and therefore the thickness of semiconductor chips obtained from the back surface grinding is kept almost constant.
In addition, it is possible for the control section to determine the timing such that the back surface grinding of the semiconductor wafer by the grinder is finished when a predetermined time elapses after the detection of a change in the air pressure inside the groove.